FIG. 1 is a sectional view of a conventional electric double layer capacitor. The electric double layer capacitor includes a pair of polarizable electrodes 1, 2 impregnated with an electrolytic solution and stacked with the intervention of a separator 3, and an electrically conductive case 4b housing the polarizable electrodes 1, 2. The upper polarizable electrode 1 is covered with an electrically conductive lid 4a. The lid 4a and the case 4b respectively contact the polarizable electrodes 1 and 2 with the intervention of electric collectors 5 and 6. The peripheral portions of the lid 4a and the case 4b are sealed with a gasket 7. Thus, the lid 4a and the case 4b are electrically isolated from each other by the gasket 7, and the electrolytic solution is prevented from leaking out of the case 4b. 
In general, the electric double layer capacitor is soldered to a printed board by the following reflow process. A solder paste is first applied onto the printed board, and the electric double layer capacitor is placed on the solder paste. The electric double layer capacitor and the board are put in a reflow oven, and heated at a high temperature on the order of 250°0 C. in the reflow oven to melt the solder paste. Thus, the electric double layer capacitor is soldered to the board.
An exemplary material conventionally employed for the gasket 7 of the electric double layer capacitor is polypropylene (PP) which is excellent in resilience, creep resistance and formability and less costly (see Japanese Unexamined Patent Publication No. 2000-232040). However, the polypropylene has a low heat resistant temperature, so that the gasket 7 is liable to be deformed by the heat of the reflow oven.
To solve the problem associated with the reflow heat, Japanese Unexamined Patent Publication No. 2002-56827, for example, proposes that a highly heat-resistant fluororesin is employed as a material for a gasket of a secondary battery and a nonaqueous electrolytic solution having a boiling point of not lower than 200° C. at an ordinary pressure is employed.
Where the fluororesin is employed as the material for the gasket 7, however, the fluororesin chemically reacts with the nonaqueous electrolytic solution at a high temperature in the reflow process depending on the kind of the electrolytic solution. This results in deformation of the gasket 7.
Further, the nonaqueous electrolytic solution is degraded by the chemical reaction, whereby the charge/discharge characteristic of the electric double layer capacitor is deteriorated.
It is therefore an object of the present invention to provide an electric double layer capacitor which includes a gasket less liable to be deformed at a high temperature in a reflow oven and is excellent in charge/discharge characteristic.